Device for rotational motion capture of a solid

ABSTRACT

The present invention relates to a capture device of the orientation of a solid comprising: 
         at least a sensor ( 10   a,    10   b ) of angular position, capable of being made solid with the solid and of supplying at least a measuring datum (θ m ) representative of the orientation of the solid,    means ( 14 ) for generating test data (θ t ) representative of an estimated orientation of the solid,    means ( 18 ) for modification of the estimated orientation of the solid by confrontation of the measuring datum and test data. Application to computer input peripherals, in the medical field.

TECHNICAL FIELD

The present invention concerns a device and a process for sensing theorientation and the rotational motion of a solid. The devices forsensing motion, sometimes designated by mocap (motion capture) areapplied in fields as varied as health, multimedia, mining or geophysicalresearch.

In the field of application of video games or simulations, the motionsof a user can in effect be registered for controlling virtual realityimmersion systems. By way of example, the motions of a player can beregistered to control the evolution of a virtual person in a synthesisscene.

In the field of health, the motion capture devices can be utilised forpositioning a surgical instrument or else for monitoring the evolutionof the autonomy of fragile persons by taking their physical activityinto account.

In the field of portable electronics, the motion capture devices allowapparatus to adapt to the context of utilisation. They allow, forexample, optimising of reception, of a portable telephone by recognisingits orientation, or improving the interfaces of personal assistants.

PRIOR ART

Motion sensors, and more precisely angular position sensors, are greatlyminiaturised and are the focus of research to impart them with arobustness and cost compatible with applications for the public atlarge.

The position of a solid in space is entirely determined by the knowledgeof six magnitudes. Examples of these are three magnitudes capable oftranslating translations and three other magnitudes capable oftranslating rotations. The three latter magnitudes correspond to angularpositions. These can be utilised to determine motions known as skewmotion, pitch motion and roll motion.

According to the envisaged applications it is not always necessary toplace all six associated magnitudes at six degrees of liberty. A morerestrained number of data can in effect suffice in a large number ofcases.

Two types of sensors are mainly known which are capable of detecting theangular position or the rotation of a solid. These are on the one handsensors sensitive to a magnetic field, such as magnetometers, and on theother hand sensors sensitive to acceleration, such as accelerometers. Inan advantageous manner the accelerometers can measure any accelerationsof the solid, thus for example, modifications of the orientation of thesolid relative to the direction of the field of terrestrial gravity.

The magnetometers can be utilised in combination with an artificialsource of magnetic field. All the same it is preferred to make use ofmagnetometers capable of detecting the orientation of the solid relativeto the terrestrial magnetic field. It is considered of course that thedirections of the magnetic field and of the acceleration of the apparentgravity are not colinear.

The sensors can be of the type having a single axis, that is, sensitiveaccording to a single direction unique of space. However, sensors havingtwo or three non-parallel axes are preferred. These supply measuringvalues allowing an angular position of a solid to which they are solidto be recognised completely.

The sensors supply a measuring signal M which is connected to theirinclination I by a function f such as:M=f(I).

The inclination is considered relative here to an angular referenceposition. The latter can be arbitrary or adjusted on the magnetic fieldor the terrestrial gravitational field. The magnitude to be known is theinclination I, which can be retrieved by calculation according to aformula I=f⁻¹ (M).

The inverse function f⁻¹ is however difficult to establish withexactitude. In addition, it suffers from discontinuities andnon-linearities. A difficulty is attached for example to the fact thatthe sensors using the field of apparent gravity do allow rotations to befound at any instant about horizontal axes but not about the directionof apparent gravity. The same applies to magnetometers which areefficacious only for measuring rotations whereof the axis is notconfused with the direction of the magnetic field used as reference.Non-linearities also originate from trigonometric functions brought upby calculating the inverse function.

Additional inaccuracies originate from the fact that the sensors havingthree axes do not always have a very precise angular relation betweenthe axes. For example, the axes are not exactly orthogonal.

An illustration of the state of the art can still be found in thedocuments (1) to (4) whereof the references are specified at the end ofthe present description.

EXPLANATION OF THE INVENTION

The aim of the invention is to propose a device and a capture process ofthe orientation of a solid not having the abovementioned limitations anddifficulties.

An aim in particular is to propose such a device which is low in costand capable of being integrated into equipment destined for a widepublic.

Another aim is to propose a reliable device, little sensitive tophenomena of non-linearity affecting measurements, and allowing possibleimperfections in the sensors to be taken into account directly.

To attain these aims the invention more precisely concerns a capturedevice of the orientation of a solid comprising:

-   -   at least one angular position sensor capable of being made solid        with the solid and of providing at least one measuring datum        representative of the orientation of the solid,    -   means for generating test data representative of estimated        orientation of the solid,    -   means for modification of the estimated orientation of the solid        by confrontation of the measuring datum and test data.

In the following description reference is made to the orientation of asolid. The solid does not however form part of the capture device.

The orientation corresponds more precisely to that of the sensor orsensors capable of being fixed to the solid. Furthermore, the termsorientation and angular position are utilised as synonyms. Owing to theinventive device it is possible to successively refine estimation of theorientation of the solid.

After one or more modifications of the estimated orientation, the latterconverges towards effective orientation of the solid, or, moreprecisely, towards measured orientation. Therefore, the inventive devicedoes not require calculation means to establish the orientation or theinclination of the solid on the base of a function (inverse) of themeasuring data of the sensors.

The inventive device allows the imperfections of the sensors to be takendirectly into account and allows non-linear behaviours of the latter tobe set free. By way of example, utilisation of sensors with threesensitive non-orthogonal axes is possible.

According to a particular realisation of the device, the modificationmeans of the estimated orientation can comprise a first comparatorconnected on one side to the sensor and on the other side to the testdata generator means. The first comparator thus receives the measuringdatum and a test datum, and can establish at least one differencebetween the test datum and the measuring datum.

The difference between the test datum and the measuring datumconstitutes measuring the pertinence of the estimated orientation.

The correlation between the estimated orientation and the test datumgenerated can be given, for example, by a direct function f as evoked inthe introductory section of the description. This is, for example, asimple function of modelling of the behaviour of the sensors.

The difference between each of the successive test data and themeasuring datum can also be put to profit to control the necessity ornot of further refining the estimated orientation. Accordingly, thedevice can comprise a second comparator with threshold for comparing thedifference established by the first comparator with a threshold valueand for validating the estimated orientation, when the differenceestablished by the first comparator for a given test value is less thanthe threshold value.

When the difference remains too significant a new estimation of theorientation is undertaken.

The modification means of the estimated orientation and/or the generatormeans of a test datum can comprise a calculator for establishing a newestimated orientation and/or a new test datum according to a methodknown as descent of error gradient.

Furthermore, the generator means of test data can comprise a calculatorfor calculating test data as a function of estimated orientation, and asa function of parameters characteristic of a response of the angularposition sensor.

The inventive device can comprise one or more angular position sensorssensitive to gravity and one or more angular position sensors sensitiveto a magnetic field.

In a more general sense other sensors are capable of giving informationon their angular position relative to a reference direction of space.

For example, there are sensors for measuring a temperature gradient, apressure gradient, image sensors (visible or thermal).

By way of example, the sensor sensitive to gravity can comprise at leastone accelerometer and the sensor sensitive to a magnetic field cancomprise at least one magnetometer.

In order to measure the angular position in the most complete manner andthe best determined, the device is preferably equipped with two sensorseach having three axes of sensitivity.

The invention also concerns a capture device of the rotation motion of asolid comprising an orientation capture device such as describedhereinabove and means for registering successive estimations of theorientation of the solid. This is, for example, a memory. The device canalso comprise a timer for rating the registration of the successiveestimations of the orientation of the solid. The timer also allowsspeeds and angular accelerations to be established, if necessary.

Calculating the motion can take place in the calculator and according toclassic laws of kinetics of a solid.

The invention further relates to a process for estimating theorientation of a solid comprising the following stages:

-   a) input of at least one measuring datum originating from at least    one angular position sensor and the establishment of at least one    test datum representative of an estimated orientation of the sensor,-   b) confrontation of the test datum and of the measured datum,-   c) establishment of at least a new test datum representative of a    new estimated orientation of the solid, corrected as a function of    the preceding confrontation,-   d) repetition of stages b) et c).

Stages b) and c) can be repeated until the confrontation reveals adifference between the test datum and the measuring datum less than adetermined threshold.

The confrontation of the data can comprise their comparison or thecalculation of a difference, as shown hereinabove.

The invention differs from the devices of the prior art by the fact thatthe determination of the orientation is not necessarily done in constanttime. In the devices of the prior art, the determination of theorientation is effected in a fixed time corresponding to the necessarycalculation time. In the case of iterative confrontation such asindicated hereinabove, the time taken by the determination of theorientation is, for example, associated with pertinence of the initialestimation of the orientation and the speed of convergence of successiveestimations. In other terms, the time given for determining theorientation depends on the number of repetitions of stages b) and c).The processing time does not however constitute an obstacle forimplementing the process. In fact the real measurements made are of theorder of 500 per sensor and per second. It is thus possible to effectseveral estimation loops for each measurement. The number of loops isgenerally less than 30. Often, a few loops suffice.

As indicated hereinabove, during stage c), a correlation calculation canbe made according to a method of error gradient descent. Even thoughthis constitutes a less preferable solution it is still possible to makerandom estimations.

Other characteristics and advantages of the invention will emerge fromthe following description, in reference to the FIGURE of the attacheddiagram.

This description is given purely by way of illustration and notlimiting.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

The references 10 a and 10 b indicate respectively an accelerometer anda magnetometer. These are sensors with three axes of sensitivity, ofknown type, capable of providing measuring data representative of theorientation, that is, an angular position, of a solid S. The solid S isindicated summarily by broken lines. This is for example a part of thehuman body, the motions of which are to be studied, a computer mouse, asurgical instrument.

The measurements of the sensors, designated θ_(m), are of scalar orvectorial magnitudes. They are representative, for example, of angles ofskew, roll and pitch (φ, ψ, θ).

These measurements are directed towards a comparator 12. This is, in theexample illustrated, a differentiator. The comparator 12 also receivesone or more test data θ_(t) provided by a calculator 14. The test datumcan be vectorial in type and can express angles according to severalaxes. The calculator 14 is utilised as means for generating test data.The test data are representative of an estimated orientation of thesolid which can be random or not. These are, for example, triplets ofangles of skew, roll and pitch (φ, ψ, θ). The calculator can belocalised on the solid S.

The comparator provides a difference ΔΘ, which, according to one or moreaxes, represents a spread between the real orientation, corresponding tothe measuring datum, and the estimated orientation corresponding to thetest datum. This spread can be used to refine the estimated orientationof the sensor, and thus of the solid to which it is fixed.

Nevertheless, it is possible to fix a threshold th beyond which it isconsidered that the estimated orientation is sufficiently close to theorientation measured to be validated. This can take place by means of asecond comparator 16 provided to compare the difference ΔΘ with thethreshold value th.

When the difference is less than the threshold in absolute value thetest datum θ_(t), that is, the estimation of the angular position, isdirected towards an output O.

However, when the difference is greater than the threshold, it isdirected towards the calculator 14 to effect a new estimation of theposition. The comparators 12 and 16 thus constitute with the calculator14 means 18 for modification of the estimated orientation of the solidS.

The new estimation can be random. It can also be refined according to acorrection calculation by the error gradient descent method. Thismethod, known per se, is illustrated by the document (4) whereof thecoordinates are specified at the end of the description and to whichreference can be made to complete the explanation.

The second comparator can optionally be eliminated. In this case, theestimated value is continually refined until the input of a newmeasurement value.

The device of the FIGURE comprises means, for example a memory, forregistering the successive estimated values, validated, as a function ofsuccessive measurements of the angular position of the solid. The memoryM can be part of the calculator and can be localised on the solid S. Thesuccessive values enable calculation of the motion of rotation of thesolid as well as its speeds and angular accelerations. To start themeasuring of a new orientation of the solid, the first test datumgenerated is advantageously the validated estimated value of thepreceding position.

The capture of measurement values by the sensors, and the registering ofthe estimated values in the memory M, can be rated by a timer H.

Contrary to the process of the known type of direct inversion, thecapture process of the orientation of a solid according to the inventionallows utilisation of any number of sensors, provided that this numberis greater than the number of variables of angle I to be estimated (thenumber of variables of angle I to be estimated is between 1 and 3).According to the desired quality of the estimation, a device accordingto the invention can thus comprise the minimal number of sensorsnecessary or a number of sensors greater than the minimal number(redundancy).

According to a refinement of the invention, the contribution of eachsensor can be weighted. A criterion of confidence or weight Cm is thenestablished which is associated with each constituent of the measurementΘm in order to take the latter more or less into account in thealgorithm of angles research. The calculation of a weight Cm isestablished according to the following rules:

-   -   a) the weight Cm has a value equal to 1 by default,    -   b) the weight Cm takes the value 0 in the event where the        provided measurement is an aberrant value (saturation, value        translating bad functioning, etc.),    -   c) the weight Cm has a value equal to 0 when the level of noise        measured by the sensor exceeds a certain threshold, an        intermediate value varying linearly from 0 to 1 able to be        applied for noise values varying from the threshold value to a        noise value considered as negligible,    -   d) the confidence is reduced on the accelerometers if the total        acceleration measured moves away in standard from the value of        apparent gravity,    -   e) the confidence is reduced on the magnetometers if the        magnetometers register an excessive variation in their standard        (the presence of a ferromagnetic object(s) in the vicinity of        the sensor can then be suspected).

In the absence of weighting, for iteration done by the calculator 14,the modification of a test angle I is associated with the magnitude S₁such as:${S_{I} = {\sum\limits_{n - 1}^{N}\left( {\alpha_{In}\Delta\quad\Theta_{n}} \right)}},$where

-   n is the index of a sensor,-   N is the number of sensors,-   α_(ln) is a parameter relative to the index sensor n, calculated    usually by the gradient descent,-   ΔΘ_(n) is the spread between the real orientation and the estimated    orientation of the index sensor n.

The introduction of a weight Cm_(n) relative to the index sensor n thenmodifies the expression of the magnitude S₁ as follows:$S_{I} = {\sum\limits_{n - 1}^{N}{{Cm}_{n}\left( {\alpha_{In}\Delta\quad\Theta_{n}} \right)}}$

In general, the values of a weight Cm_(n) can evolve continuouslybetween the value 1 (total confidence in the measurement made by theindex sensor n) and the value 0 (total absence of confidence in themeasurement made by the index sensor n, the measurement made by theindex sensor n is not taken into consideration).

LITERATURE

-   (1) U.S. Pat. No. 5,953,683, “Sourceless orientation sensor” by    Kogan Vladimir et al.-   (2) U.S. Pat. No. 6,702,708, “A miniature, sourceless, networked,    solid state orientation module” by Christopher Townsend et al.,    MicroStrain Inc. 294 N. Winooski Ave., Burlington, Vt. 05401, USA-   (3) “A miniature, sourceless, networked, solid state orientation    module”, by Christopher Townsend, David Guzik, Steven Arms,    MicroStrain Inc., 294 N. Winooski Ave., Burlington, Vt. 05401, USA,    pages 44 to 50.-   (4) “Methode de calcul numerique” [Numerical calculation method]    by J. P. Nougier, 3rd edition 1987, Edition Masson, pages 54-58.

1. Capture device of the orientation of a solid comprising: at least asensor (10 a, 10 b) of angular position, capable of being made solidwith the solid and of supplying at least a measuring datum (θ_(m))representative of the orientation of the solid, means (14) forgenerating test data (θ_(t)) representative of an estimated orientationof the solid, means (18) for modification of the estimated orientationof the solid by confrontation of the measuring datum and test data. 2.Device as claimed in claim 1, wherein the modification means (18) of theestimated orientation comprise a first comparator (12) connected to thesensor (10 a, 10 b) and to the means (14) for generating test data, forreceiving the measuring datum and at least a test datum, and forestablishing at least a difference (ΔΘ) between the test datum and themeasuring datum.
 3. Device as claimed in claim 2, further comprising asecond comparator with threshold (16) for comparing the differenceestablished by the first comparator (12) to a threshold value (th) andto validate the estimated orientation, when the difference establishedby the first comparator is less than the threshold value.
 4. Device asclaimed in claim 1, comprising at least an angular position sensor (10b) sensitive to the gravity and at least an angular position sensor (10a) sensitive to a magnetic field.
 5. Device as claimed in claim 4,wherein the sensor sensitive to gravity comprises at least anaccelerometer and the sensor sensitive to a magnetic field comprises atleast a magnetometer.
 6. Device as claimed in claim 4, comprising twosensors each having three axes of sensitivity.
 7. Device as claimed inclaim 1, wherein the means (14) for generating test data comprise acalculator for calculating test data as a function of an estimatedorientation, and as a function of parameters characteristic of aresponse of the angular position sensor.
 8. Device as claimed in claim7, wherein the calculator is localised on the solid.
 9. Device asclaimed in claim 1, wherein the modification means (18) of the estimatedorientation and/or the means for generating a test datum comprise acalculator for establishing a new estimated orientation and/or a newtest datum according to a method known as error gradient descent. 10.Device as claimed in claim 9, wherein the calculator is localised on thesolid.
 11. A motion capture device of the rotation of a solid comprisinga capture device of the orientation, as claimed in claim 1 and means (M)for registering successive estimations of the orientation of the solid.12. Device as claimed in claim 11, wherein the means (M) for registeringare localised on the solid.
 13. Device as claimed in claim 11,comprising a timer (H) for rating the registration of the successiveestimations of the orientation of the solid.
 14. A process forestimation of the orientation of a solid comprising the followingstages: a) capture of measuring data originating from at least oneangular position sensor (10 a, 10 b) and the establishment of a testdatum representative of an estimated orientation of the solid, b)confrontation of the test datum and the measured datum, c) establishmentof a new test datum representative of a new estimated orientation of thesolid, corrected as a function of the preceding confrontation, d)repetition of stages b) and c).
 15. Process as claimed in claim 14,wherein the stages b) and c) are repeated until the confrontationreveals a difference between the test datum and the measuring datum lessthan a determined threshold.
 16. Process as claimed in claim 14,wherein, during stage c), correction calculation is made according to amethod known as error gradient descent.
 17. Process as claimed in claim14, wherein confrontation test data and the measuring datum comprisesthe establishment of difference data (ΔΘ) between successive test dataand the measuring datum.
 18. Process of motion capture of a solid,characterised in that the process as claimed in claim 14 is repeatedwith successive measuring data.